This invention relates generally to pigments and more specifically relates to a process for producing high brightness pigments by beneficiation of naturally occurring calcium carbonate.
Calcium carbonate pigments find application in a wide variety of industrial and other environments. Such pigments, for example, are widely utilized as fillers in the manufacture of papers, rubber and various plastics, and as extenders in paint formulations. Such pigments furthermore, either alone or in combination with other pigments, are widely utilized for paper coating applications. In many of the aforementioned applications, particularly e.g. where the pigments are used for coating, it is desirable that the calcium carbonate be as bright as possible.
High brightness calcium carbonate pigments have long been produced by chemical processes, pursuant to which the said carbonates are prepared as precipitates. These processes, however, are comparatively complex and are not well suited to large scale or low cost operations. Interest for such reason has long centered upon possible use of naturally occurring calcium carbonate, particularly in that natural calcitic minerals are extremely abundant in virtually all parts of the world, and therefore represent a ready source of inexpensive raw material. In practice, however, it is found that many of the natural calcitic deposits are so highly contaminated with discolorants, that when comminuted in their natural state they are simply inacceptable as pigments. Thus in typical instances deposits which are primarily of calcite may be contaminated with pyrites and with mica, both of which in varying degrees contribute to the discoloration of the otherwise relatively colorless calcium carbonate.
Various proposals have from time to time been advanced for processes directed at improving the brightness of the aforementioned naturally occurring calcium carbonates. Many of these prior proposals involve the use of flotation methods to remove impurities from calcitic ores. The bulk of such prior art, however, is centered upon the removal of siliceous impurities, so as to improve the remaining composition for use in cement manufacture.
In some instances too, flotation is recommended for improving the brightness characteristics of naturally occurring calcium carbonate. Thus, in U.S. Pat. No. 3,512,722, a wet-ground natural calcium carbonate is subjected to a flotation, after which the underflow is classified, partially dewatered, dried, and pulverized to reduce the agglomerates and larger particles.
Additional prior art pertinent to processes for brightening natural calcitic ores, is also cited in U.S. Pat. No. 3,980,240, which patent is assigned to the assignee of the present application. The method disclosed in that patent is thus concerned with brightening of a natural calcitic ore by crushing and forming same into an aqueous slurry, coarse-milling the slurry, fine milling the slurry to alter the particulates such that at least 70% of the particles are less than 2 microns, and then subjecting this fine-milled particulate to a separation by application of a high intensity magnetic field and/or by use of flotation.
Except for the aforementioned U.S. Pat. No. 3,980,240, the techniques of the prior art as discussed above, have in general been of limited efficacy in producing high brightness calcium carbonate pigments from natural sources. Indeed, in most instances where pigments are derived on a commercial scale from such natural sources, an ore is utilized which to begin with is of very high purity and relatively free from discoloring contaminants. Reference may be had in this connection, for example, to U.S. Pat. Nos. 3,661,610 and 3,674,529, which cite the use of calcium carbonate pigments derived from a raw, high purity natural chalk whiting. The natural whiting is subjected to a two-stage grinding process, it not being necessary in these instances to subject the product to purification, i.e. separation steps.
While the processes of the aforementioned U.S. Pat. No. 3,980,240 patent are in general most effective, it is yet found that certain disadvantages occur where one seeks to beneficiate the finely ground carbonate, i.e. the output from the second, fine grinding stage. Since, in particular, such beneficiation steps are normally carried out at relatively low solids (typically less than 50% by weight solids), problems can develop in the subsequent step of removing water from the slurry. In particular it is found to be quite difficult to dewater slurries of fine ground carbonates (e.g. where the particulate is such that 80% or more by weight of the particles are less than 2 micron) to achieve a solids content at which such slurry can be commercially shipped--i.e. preferably greater than 70% solids. Furthermore, as will be considered further hereinbelow, it is found that when grinding is carried out to a very fine degree, i.e. when the particulates are reduced to a very fine stage of comminution prior to separation, the colorbodies in consequence are so thoroughly and evenly distributed, throughout the material from which they are sought to be removed, that the efficacy of flotation is to a degree impeded, and the recovery of saleable product from the flotation step is much reduced.
In acccordance with the foregoing, it may be regarded as an object of the present invention, to provide a process which enables the production of high quality bright calcium carbonate pigments from naturally occurring calcitic ores containing relatively high levels of discoloring contaminants.
It is a further object of the present invention, to provide a process for purifying and brightening natural calcitic ores of relatively high discoloration, to yield a slurried pumpable product of high solids content, wherein the slurried carbonate includes at least 80% by weight of particles less than 2 microns E.S.D., and wherein the resultant product brightness is at least 94.
It is a still further object of the present invention, to provide a process meeting the foregoing objectives, which, further, enables beneficiation without adverse effects upon the subsequent dewatering operations which are deemed desirable to produce a product in a commercially advantageous form.